Responses of different body compartments to acute dietary phosphorus deficiency in juvenile triploid rainbow trout (Oncorhynchus mykiss , Walbaum)

Abstract

Rainbow trout (Oncorhynchus mykiss) is an important production species as well as one of the most studied fish models, particularly regarding nutritional physiology. Due to negative environmental impacts linked with rainbow trout farm effluents, significant restrictions have been established in numerous regions to reduce dietary phosphorus (P) outputs. However, questions have arisen regarding the link between abnormal skeletal development and mineralization and insufficient dietary P availability during rapid fish growth (juvenile fish). Despite significant work to understand the dynamics of P-deficiency and the resulting impact on tissue mineralization, the extent of the early responses in rainbow trout fed low-P diets is not well described. The aim of this study was to explore the early-responses of scales, vertebrae and carcass P and ash in rainbow trout fed low-P vs sufficient-P controls. Two practical diets (sufficient: 0.45% available P and deficient: 0.29% available P) were fed over a 5-week period to triploid rainbow trout (initial mass 60.8 ± 1.6 g). Ash and P contents were used to assess mineral status of the different tissues. The relative loss of mineral and coefficient of variation were also calculated to compare the relative response and the inter-individual variability. After 4 weeks of P deprivation, no detectable effects were observed on growth performance, mortality or feed intake. However, as early as the second week onward, ash and P levels in scales and carcasses were significantly lower (p < 0.05) in fish fed a P-deficient diet compared to those fed a P-sufficient diet. At week 4, the reduction in mineralization reached 19.3 and 18.4% for P and ash contents in scales, respectively. By the same week, P and ash contents in carcasses reached 15.1 and 12.8%, respectively. Interestingly, the absence of significant mineral loss in vertebrae did not prevent the emergence of a high incidence of vertebral deformities in P-deficient fish (45.6 ± 11.0%) when compared to P-sufficient fish (1.5 ± 2.1%). The main vertebral deformities observed in P-deficient trout were specific to impede bone matrix mineralization such as pronounced biconcave (35.5%) and undersized and widely spaced vertebral bodies (9.3%). For the scales compartment (ash and P status) and mineral parameter, the coefficients of variation showed a higher inter-individual variability in P-deficient vs P-sufficient fish, while no such effect was observed in carcass or vertebrae compartment. This study provides a useful comparison of various commonly-used indicators of bone mineral status as well as an approach to assess variability of fish response under low-dietary P level.